1. Field of the Invention
This invention relates to a multi-layer ceramic substrate and a method for producing such substrate. More particularly, the invention pertains to a multi-layer ceramic substrate suited for constituting a function module by mounting semiconductor parts and joining pins for signal input and output thereto, and a method for producing such a substrate.
2. Description of the Prior Art
Recently, ceramic substrates incorporating signal lines in multiple layers are used popularly as the substrate for mounting semiconductor parts in a high-density arrangement. The ceramic insulators used for such substrates are mostly made of a sintered body principally composed of alumina (Al.sub.2 O.sub.3). A typical example of such ceramic insulators is discussed in a paper "A Multi-Layer Ceramic, Multi-Chip Module" (IEEE, 30th Electronic Component Conference, 1980, pp 283-285). The greatest disadvantage of the substrate using such an alumina-based sintered body as an insulator is the large signal propagation delay. Generally, the signal propagation delay is proportional to the square root of the dielectric constant of the insulator surrounding the signal lines, and the dielectric constant of alumina is as large as about 10, so that the sintered body which is mainly composed of alumina also has a large dielectric constant which is on the order of 9.0 to 10.0. Such a large dielectric constant of the sintered body leads to a large signal propagation delay of the substrate.
For solving the problem of large signal propagation delay, studies have been made on the substrates using an insulator with a small dielectric constant. An example of such substrates developed recently is disclosed in Japanese Patent Laid-Open Publication No. 57-115895 "Mullite sintered body and method for the production thereof". This patent shows that the use of a sintered body mainly composed of mullite (3Al.sub.2 O.sub.3.2SiO.sub.2) as an insulator can give a ceramic substrate with a dielectric constant as low as 5.4 to 5.9.
Inspite of such advantages of mullite, alumina is still dominantly used in the field of multi-layer ceramic substrates. This is due to the fact that it is impossible with mullite to obtain as high thermal and mechanical strength as provided by the use of alumina, and that the substrates using mullite tend to suffer from crack, wear-off of the metallized surface coating and other trouble originating in the stress built up by the heat generated in the step of soldering pins and other parts to the substrate, making it difficult to obtain the products of high reliability in a commercial yield.